As well known to those skilled in the art, sensory signal output apparatuses are classified into sound generating apparatuses, such as speakers, receivers and buzzers, which output voices and sounds, and vibration generating apparatuses, which generate vibration force as well as sounds to output sounds in multiple frequencies.
Here, a speaker (or a receiver) is an apparatus that converts an input frequency signal into sound and outputs the sound. In the speaker, electric energy is converted into mechanical energy by a voice coil, provided in an opening defined in the speaker, using Fleming's left-hand rule, in which, when a conductor through which electric current flows is placed in a magnetic field, a thrusting force is applied thereto.
In detail, when current signals including several frequencies are applied to the voice coil, the voice coil generates mechanical energy depending on the intensity of electric currents and the magnitude of frequency and vibrates a vibrating screen attached to the voice coil, thus generating sound pressure waves having a predetermined frequency perceptible by the ears of humans.
Of such apparatuses for generating sound pressure, an apparatus that generates relatively low sound pressure and is used in a state of being close to the ear of a human is called a receiver, and an apparatus that generates relatively low sound pressure and is used in a state of being spaced apart from the ear of a human by a predetermined distance is called a speaker.
In the speaker, a magnetic circuit is constructed such that magnetic flux linkage, perpendicular to a voice coil placed in an opening, is induced using a magnet (a permanent magnet) and a top plate in a yoke made of iron metal. The voice coil, which is adhered to a vibrating screen, generates excitation force in response to an input signal, and thus vibrates the vibrating screen, which is attached to a frame, thus generating sound pressure.
The vibrating screen has various shapes of waves to ensure superior responsiveness and to prevent a buckling phenomenon when it vertically vibrates. This shape of the vibrating screen acts as a critical design variable that influences frequency characteristic.
Such speakers have not greatly changed in structure for many years, but, recently, thanks to the commercialization of high-energy permanent magnets and the development of techniques for forming fine structures, and to satisfy the trend towards lightness and compactness in the information and communication field, lightness and compactness and high performance of speakers have been realized. Despite developments in the technique, the compactness of electric-acoustic transducers and electronic devices having the electric-acoustic transducers limits the output of speakers. This remains as a technical problem in this art.
Furthermore, in the case of a speaker device, which outputs sounds at several levels, that is, low tones, middle tones and high tones, to improve the quality of sound, because several speakers are installed in a cabinet having a box shape, there is a disadvantage in that the volume thereof is increased.
In addition, recently, an electric-acoustic transducer, in which two magnetic circuits and vibration structures are provided in a single frame, was proposed. However, in the conventional electric-acoustic transducer, because vibration structures are oriented in opposite directions, when the vibration structures are operated at the same time, the vibrating forces of the vibration structures offset each other. As a result, there is a problem in that the efficiency of output, relative to an input signal or to the amount of power consumed, is reduced.